For example, in motorcycle engines, a construction in which a crankcase and a cylinder body are provided separately for connection to each other with connecting bolts or a construction in which a cylinder head is connected to a crankcase via a cylinder body interposed therebetween with connecting bolts is adopted in general.
Incidentally, a crankshaft is disposed below a mating surface of the crankcase, with which the cylinder body is brought into mating contact for connection, in such a manner as to pass through left and right side walls of the crankcase, and a great magnitude of load is applied to portions of the crankcase where crankshaft bearings are disposed due to combustion pressure. In general, crankcases are made of aluminum alloy, and in order to secure a rigidity which can bear the great magnitude of bearing load, there exist dimensional limitations such as the thickness of the crankcase and the diameter of the bearing portion.
Then, there has been proposed a construction in which a cast-iron bearing member for supporting a crankshaft bearing is embedded in a crankcase through insert casting (for example, refer to JP-A-8-21299).
Incidentally, in the conventional technique described in the Japanese unexamined patent publication, a construction is adopted in which a bearing portion of the bearing member is formed into a semi-circular shape, and a semi-circular cap member is fixedly bolted to the bearing portion with bolts, so that the crankshaft is supported by the cap member and the bearing portion. On the other hand, due to this construction, the bearing load applied by the combustion pressure acts in a direction in which the cap member is pulled away from the bearing portion. Consequently, a large load is eventually applied to the bolts that fasten the cap member, and as a result, the effect of the improved rigidity resulting from the insert casting the cast-iron bearing member in the crankcase is limited by the strength of the bolts.
The invention was made in view of the problem inherent in the conventional construction, and an object thereof is to provide an engine fastening structure which can largely improve the rigidity against the bearing load by embedding a bearing member in a side wall of a crankcase and which can largely improve the connecting force for connecting the crankcase and a cylinder body or a cylinder head.